Given they are -basically- both UE2.5, it shouldn't be that hard to cross-import them.. albeit it might be in poor taste. I blieve there are several modifications RO did to a few package formats to enhance the engine.. nothing to fancy.
Anything that won't straight-up-import into UT2k4, you'll have to export, then yes, maybe re-import via ActorX from Max/Maya/XSI/whatever.
The only thing I'll remind you to do is at minimum give proper credit to the original smesh creators, or ideally, ask the creators of those meshes for the right to use it in another product.